Wiring central heating so fire stat can select zones

[l60mcm]

Hi I am not 100% sure this is in the correct location but It is somewhat to do with wiring.

Below is the current wiring for my heating, it consists of.
1x 4 zone time clock
3x Room stats
3x Motorised valves
1x Tank stat
1x Fire stat

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• l60mcm
• 11 Dec 2013
• 1

As you can see from the above diagram at present if the fire tries to over boil the hot tank the fire stat automatically opens zone 3 and turns on the radiator pump until the temp lowers

The fire stat also deactivates the time clock zone 3 from being able to call the boiler for heat
I am thinking of doing away with this as shown below (relay 1 removed from above diagram), because in theory Z1, Z2 and HW can still call for the boiler but nothing should happen because the tank stat will know the tank is hot enough.

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• l60mcm
• 11 Dec 2013
• 1

My issue is I want to be able to manually (eventually automated with 3 channels of a cbus relay) select which zone the fire dumps to but there always has to be one zone on.

There are a number of issues with just adding a 3 gang switch like above diagram.
1 All can be accidentally left off
2 If two or more are on and the time clock comes on under normal conditions for one zone the other zones will also come on.

I am sure with a bit more configuring I will be able to work it out but I seem to be stumped at the minute, I have drawn a few other options with relays and the likes but I keep running into issues. I am sure there is a simple answer that I am just over looking

Any help will be great

Thanks

 

[EFLImpudence]

Any good? Three more relays.
From and in addition to your second diagram.

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• EFLImpudence
• 11 Dec 2013
• 1

This is just in answer to your question and may give you some ideas.
I do not know whether it will affect any safety systems.

 

[aesmith]

What sort of boiler, oil, gas or solid fuel? It's not clear how your plumbing is laid out to mean you need to "dump" heat.

 

[bernardgreen]

When you modify the system you MUST consider all aspects of safety.

One worry I would have is in the case of a power cut and therefor no pump to dump heat to the radiators would the tank then over heat.

Especially as in a power cut the fire may be the only form of heating so might be well stoked up.

Dumping heat by running hot water taps has to be done manually unless a non electric dump valve is fitted.

 

[l60mcm]

Sorry I should have explained the current setup better. There is currently a maxi pod combination type (as picture below) heat store tank. I have a kerosene condensing boiler, solar tubes and a coal fire. Currently if the fire did over heat the tank it does vent into a tank in the roof space ( I know this works because when setting up the system the fire stat was set to high and the tank vented)

When I saw dump I just mean circulate the excess heat that the heat store can not store

Thanks for the help so far

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• l60mcm
• 12 Dec 2013
• 1

 

[SimonH2]

The obvious question is why the room stats can call the boiler at all. If you run the heating from the store, then you simply use the switches on the zone valves to call up the pump and draw heat from the store. The boiler is not involved - electrically or hydraulically - at this point. Only if the other heat sources are insufficient does the tank cylinder stat call for the boiler (and it's separate pump) - and this won't happen if the fire is overheating, assuming the system is correctly designed.

So your safety design now becomes one of "how to open zone valves and call the pump if the tank is overheating". That can be a simple stat that overrides the controller and stat for one zone - or it can pull in a relay so it can open more than one zone valve. The act of opening a zone valve will turn on the pump.

Next issue is if you have TRVs on all rads - if you don't, then why not ? Potentially you could have a situation where you turn on the heat dump, but sod all happens because all the TRVs are shut. One suggestion I recall reading (can't remember if it was here or another forum) involved having a 3rd connection to some rads, so the heat dump circuit could bypass the TRV - but you might need some non-return valves to prevent "interesting" reverse flows when the heat dump loop wasn't in use. You'd add an extra zone valve to control this heat dump circuit, so no electrical complications.
My biggest concern if you had a separate zone valve like that would be that it won't be used very often - so might be prone to sticking and not opening when needed.

 

[l60mcm]

Hi thanks for your response

The obvious question is why the room stats can call the boiler at all. If you run the heating from the store, then you simply use the switches on the zone valves to call up the pump and draw heat from the store. The boiler is not involved - electrically or hydraulically - at this point. Only if the other heat sources are insufficient does the tank cylinder stat call for the boiler (and it's separate pump) - and this won't happen if the fire is overheating, assuming the system is correctly designed.

That is the way my system is wired at present. (First diagram)
1 The time clock turns on the zone
2 Room stat checks if zone is cold
3 Zone valve opens and water pumps from heat store
4 If tank stat is too cold it calls boiler (I know the fire should never call the boiler that is why I am going to do away with relay 1 in the first diagram

So your safety design now becomes one of "how to open zone valves and call the pump if the tank is overheating". That can be a simple stat that overrides the controller and stat for one zone - or it can pull in a relay so it can open more than one zone valve. The act of opening a zone valve will turn on the pump.

That is what happens at present
1 Fire stat detects store is to hot
2 Opens zone 3 and dumps heat until temp cools
3 At present it also activates relay 1 which deactivates the ability for the boiler to fire(I am doing away with this relay as it is not needed because in this situation the water temp will be higher than the tank stat so it can't be on anyway)


Next issue is if you have TRVs on all rads - if you don't, then why not ? Potentially you could have a situation where you turn on the heat dump, but sod all happens because all the TRVs are shut. One suggestion I recall reading (can't remember if it was here or another forum) involved having a 3rd connection to some rads, so the heat dump circuit could bypass the TRV - but you might need some non-return valves to prevent "interesting" reverse flows when the heat dump loop wasn't in use. You'd add an extra zone valve to control this heat dump circuit, so no electrical complications.
My biggest concern if you had a separate zone valve like that would be that it won't be used very often - so might be prone to sticking and not opening when needed.

I do have TRV valves on the rads (do you know what temp a room has to be with the valve up full for it to close.

All I am looking to achieve is the ability to select which zone the fire dumps too, but there always has to be one active.

I know I could just turn on the time clock for that zone when the fire is lit.

I could achieve it by adding 3 relays and 3 gang switch but I need to figure out how to make sure there is always one zone open

 

[swampy2]

Use 2 gang switch, 2 mv in 1 of 2 way switch , 1 of the 2 mv is always connected.

 

[SimonH2]

Sorry, I didn't quite follow your initial explanation - I see know what you have. Referring to Fire Stat made me think this was on the fire rather than being a tank overheat stat.
Ouch, that relay 3 is over complicated - it doesn't need a lot of what's there ! Doesn't need to disconnect MV3 from it's room stat - just apply power to MV3. It doesn't need to turn on the pump as the MV will do that when it opens - and if it doesn't open then running the pump won't do any good. And as you say, it's unlikely it needs to disable the boiler as the store heat stat will have taken care of that.

No need for 3 relays. Just one with multiple contacts.
As a simple option, just re-wire the existing relay with one normally open contact which applies power directly to the motor in each MV - bypassing the controller and stat.

For switching, there are a number of options. If you use a single pole 3 way switch then you can select exactly one (no more, no less) zones to be connected to a single relay contact. Or you can use two off changeover switches (if you want to use "standard" domestic fittings) to do the same thing - relay contact to Com on first switch, L1 to MV1, L2 to Com on second switch, L1 to MV2, L2 to MV3.
If you "build your own" then you can select a cam switch to give any combination you want - you'd want 7 positions to select all combinations other than "all off".

But are you over-complicating things ? Wouldn't it be simpler to just turn on all zones ? Or is this a case of trying to dump heat (and make rooms uncomfortably warm) into uninhabited areas (eg bedrooms during day, living room in early hours of morning) ? I wonder whether it would really be that practical.

 

[l60mcm]

Thanks for all your help that gives me plenty to think about.

I will probably go with something similar to below

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• l60mcm
• 19 Dec 2013
• 1

 

[SimonH2]

I do have TRV valves on the rads (do you know what temp a room has to be with the valve up full for it to close.

I'd not picked up on what you wrote there - "quick" reading again.

IME they can close off quite well when a room is up to temp - which is the case you need to design for. I'd suggest that having to manually adjust the TRVs in order to get a heat dump isn't a very satisfactory state of affairs.

I think you are over-complicating things and still haven't explained why you want to be able to select just one zone - and why you think it's a good idea. Given that this is a safety feature to avoid boiling the store, wouldn't it be simpler to configure the overheat stat to just open all the zones and "forget about it" ? In practice, if the house is already being heated then it's not going to make a huge difference if you turn on a zone that's already on - the TRVs will be throttled down and you won't take much (if any) more heat out of the system.

If the reason is "I don't want to overheat part of the house" then your priorities are wrong. The overheat stat is there to remove excess heat from the store and avoid danger<period>. If that makes some rooms warmer than you'd like then that's the cost of using the house as a heat dump - either throttle back the wood burner, open some windows, or install a heat dump that doesn't heat the inside of the house.